Evaluation of Age- and Sex-Dependent Rates of Tag Loss in Southern Elephant Seals

Abstract

Rates of tag loss were estimated in a long-term tagging study of southern elephant seals (Mirounga leonina) to assess the potential for bias in estimates of survival rates. Dalton Jumbo Rototags® were applied to each hind flipper of 5,743 recently weaned elephant seal pups on Marion Island from 1983 to 1993. We adapted and developed a method based on the resighting times of seals retaining 1 or 2 tags to estimate tag loss and test for effects of age and sex of the seals. Tag loss by young seals was low, but there was a strong increase in tag loss with seal age, especially for males. Annual single tag loss at age 14 was 10% for males and 5.6% for females. Although these are relatively modest rates of tag loss, substantial fractions of seals (35% of males and 17% of females) would lose both tags by age 15, requiring corrections to avoid bias in demographic studies based on these tagging data. The method we used to estimate tag loss has significant advantages over a ratio estimator that has been used for most previous studies of tag loss in pinnipeds.